DPD (1) is an enigmatic molecule. It has been known since 1971 as the product of catabolism of S-adenosylhomocysteine in many bacteria (Duerre et al. 1971); and, more recently, it is proposed to be the core molecule from which all bacterial AI-2 signaling molecules are derived (Miller et al. 2004). These molecules are widely used in inter-species communication in the bacterial world (Federle et al. 2001). It is a simple molecule, but was reported to be quite unstable toward rearrangement (Winzer et al. 2002; Slaughter et al. 1999) and oligomerization (Semrnelhack et al. 2004; Meijler et al. 2004), and has only recently been synthesized and tentatively characterized in dilute solution (Meijler et al. 2004).
FIG. 1A shows that DPD can exist as an equilibrium mixture of three isomers (1,2, and 3), hydrated versions (4 and 5), and borate complexes (e.g., 6), collectively known as autoinducer 2 (AI-2) (Miller et al. 2004). V. harveyi recognizes the 2,3-borate diester (6) of the hydrated α-anomer (4) of DPD as AI-2 (Chen et al. 2002). S. typhimurium recognizes the hydrated β-anomer (5) of DPD without borate (Miller et al. 2004). The incorporation of borate into the AI-2 signal may be a function of the natural habitat of the bacteria. V. harvyei is a marine bacterium that lives in an environment with highborate concentration compared to S. typhimurium, an enteric bacterium.
An efficient synthesis strategy for DPD allows characterization of DPD isomers, hydration processes, and boron complexation. The availability of much larger quantities of DPD than previously obtainable (either enzymatically or by a less efficient synthetic route should facilitate understanding DPD's role in bacterial quorum sensing for many species.